Even an experienced speaker
like Guy Kawasaki says, “Moderating a panel is
deceptively hard--harder, in fact, than keynoting."
This blog offers tips and examples on how to be a better
panel discussion moderator, with advice from The Eloquent Woman's Guide to Moderating Panels and speaker coach Denise Graveline.

This practice is so common that it's on my list of the 9 times you should turn down an offer to moderate a panel, and I recommend you let the organizers know your reason, so this practice will be stopped someday. So I welcome Gender Avenger's attention to this problem. On GA, Soraya Membreno writes:

A moderator of a panel or debate serves as a champion of the audience. The panelists, on the other hand, speak for an industry and are brought in as experts to offer insight the audience does not have. Although moderators are undoubtedly important, placing the bulk of the women in your conference in this role doesn’t actually address gender balance, and does little to promote women’s voices. So while I would never discourage a woman from serving as a moderator, it bears stating: it is not enough.